In microscopy, the magnification factor can typically be adjusted by rotating the nosepiece to use objective lenses of different magnifications in, by switching to tube lenses of different focal lengths, by adding (at least in front of the tube lenses) afocal subsystems set to infinity, by using a pancratic (afocal) system in the stand, or by digital zooming.
Rapid rotation of nosepiece for changing objective lenses, switching to tube lenses and adding afocal subsystems can result in severe fluctuations in the stand, abrupt changes in image brightness, and focus drift. Moreover, a change of the objective lens in the nosepiece frequently also necessitates a change of the immersion medium, which requires the microscopist to immerse the preparation or to remove the preparation from immersion and then relocate the position of the specimen.
Although the use of a pancratic system requires much greater structural space in the stand, digital “zooming”, in which “zooming” is defined as a continuous change in magnification, merely changes the magnification without adjusting the resolution.
The first aspect relates to compactness. All zoom systems have variable air gaps for adjusting the beam penetration depth of the aperture beam and the main beam.
The aperture beam is the beam that proceeds from the axial object point and strikes the rim of the aperture, whereas the main beam is defined as the beam that proceeds from the highest field point and travels through the center of the pupil.
Therefore the focal length (that is to say, magnification) and not the back focal length (focus) is changed. Image defects are optimized as a result.
Variations in the air gaps result in an increase in the structural sizes of zoom systems, particularly with a high zoom factor.
However, compactness (that is to say, the shortest possible structural length) is always desirable in a stand designed to benefit customers.
U.S. Pat. No. 6,674,582 specifies a multiplicity of embodiment examples of zoom objective lenses 10×/0.25 to 40×/0.60 (0.80) made by Olympus. These are intended for theoretical studies and not for manufacturing. Disadvantages of these solutions include the following:                the objective lenses have a built-in aperture diaphragm, the diameter of which must be adjusted dependent on the magnification,        the objective lenses have 16 to 20 lenses, a number of which have a plurality of aspherical surfaces and        the structural length is relatively large.        
The objective lenses can be produced only at extremely high cost, if they can even be manufactured at all.
The continuous change in magnification (zooming) is a significant aspect. In all magnification ranges, the correction of image defects is diffraction limited. Focal positions are also maintained during zooming. This enables viewers to constantly keep the position of interest (region of interest) of the specimen in view during zooming and thereby avoid image losses.
Even if the focal distance remains constant for all magnifications in the design phase, the tolerance values result in fluctuations in the focal distance which are dependent on the respective magnification. The fluctuations are subsequently measured and recorded for each objective lens. The data are then transferred to the stand. When the magnification of an objective lens is changed, it can then be automatically refocused with the help of these data, as long as the stand supports such a function.
No published optical design exists as yet for microscope objective lenses that have a zoom function and also have the following properties:                structural length≤63.56 mm,        semi-apochromatic correction without asphere,        continuous magnification adjustment with 4× zoom,        numerical aperture up to 0.8 and        number of lenses≤14.        
The structural length is the length from the object plane to the front diaphragm, while the parfocal length is defined as the length from the object plane up to the objective lens mounting surface.
For microscopy, zoom objective lenses having a parfocal length of 45 mm and a structural length of 48.56 mm and having a cover glass of 0.17 mm are not feasible. The next ISO standardized stage for parfocal length would be 60 mm with a structural length of 63.56 mm and a cover glass of 0.17 mm.
In view of the disadvantages of the prior art, the object of the invention is to improve upon a zoom objective lens for microscopy such that the cost of producing the objective lens with a minimal structural size is decreased, while the working distance remains constant, so that the object always remains in focus with a change in magnification.